1. Field of the Invention
The present invention relates generally to methods of cementing high temperature wells and cement compositions therefor, and more particularly, to such methods and compositions which are suitable for cementing high temperature wells containing carbon dioxide.
2. Description of the Prior Art
In the completion of high temperature subterranean wells and particularly in such wells containing carbon dioxide, e.g., geothermal wells, the use of conventional hydraulic cement compositions often results in early well failure. Because of the high static well bore temperatures involved often coupled with the presence of brines containing carbon dioxide, conventional hydraulic well cements rapidly deteriorate due to alkali carbonation, especially sodium carbonate induced carbonation. In geothermal wells which typically involve very high temperatures, pressures and carbon dioxide concentrations, conventional well cement failures have occurred in less than five years causing the collapse of the well casing.
It has heretofore been discovered that a cement material known as calcium phosphate cement formed by an acid-base reaction between calcium aluminate and a phosphate-containing solution has high strength, low permeability and excellent carbon dioxide resistance when cured in hydrothermal environments. It has also heretofore been discovered that a cement composition formed of calcium aluminate, fly ash, sodium polyphosphate and sufficient water to form a pumpable slurry has excellent strength and carbon dioxide resistance when cured in hydrothermal environments. While this cement composition has achieved success in cementing high temperature wells including those containing carbon dioxide, the cement material has been subject to high fluid losses when placed in the wells.
Thus, there are needs for well cement compositions useful in cementing high temperature wells including those containing carbon dioxide which have low fluid losses.
The present invention provides improved methods and cement compositions which meet the needs described above and overcome the deficiencies of the prior art. The methods and cement compositions are useful in high temperature wells and are particularly useful in high temperature wells containing carbon dioxide such as geothermal wells.
The methods of the present invention for cementing a high temperature subterranean zone penetrated by a well bore basically comprise the steps of forming a well cement composition of this invention, pumping the cement composition into the subterranean zone by way of the well bore and allowing the cement composition to set into a hard impermeable mass therein.
A cement composition of the present invention is basically comprised of calcium aluminate, fly ash, sodium polyphosphate, a fluid loss control additive, and sufficient water to form a pumpable slurry. When required, a set retarder can also be included in the composition.
The objects, features and advantages of the present invention will be readily apparent to those skilled in the art upon a reading of the description of preferred embodiments which follows.